The present invention relates to ceramic bodies formed from yttria stabilized zirconia-alumina having particular utility as molten metal filters.
Molten metals in practice generally contain entrained solids which are deleterious to the final cast metal product. These entrained solids usually derive from three sources. Some are oxide particles which are drawn into the liquid stream from an oxide layer floating on its surface. Others are fragments of furnace lining, transfer trough and other portions of the molten metal handling equipment which are eroded and entrained in the flowing molten metal stream. Still other particles are precipitates of insoluble impurities such as intermetallics, borides, carbides, or precipitates of other metal compounds such as chlorides. When these inclusions appear in the final cast product after solidification, they often cause a decrease in ductility or poor finishing characteristics. Accordingly, it is desirable to remove entrained solids from the molten metal stream before it is cast into a solid body which may be used as such or subjected to forming operations such as rolling, forging, extrusion, etc.
Filtering processes to remove entrained solids from liquids are accompanied by passing the solid-laden liquid through a porous filter medium that will not pass the solids. Filtering molten metal creates special problems because the liquid is so aggressive that it is difficult to find filter mediums capable of withstanding it.
Ceramic foam filter bodies having an open cell structure characterized by a plurality of interconnected voids surrounded by a web of ceramic are employed to remove entrained solids from molten metal. These bodies have been formed from a variety of ceramic materials including alumina-chromia and zirconia-alumina compositions. U.S. Pat. Nos. 3,893,917 to Pryor et al., 3,947,363 to Pryor et al., 4,024,212 to Dore et al., 4,265,659 to Blome, 4,343,704 to Brockmeyer and 4,839,049 to Kinney, Jr. et al. illustrate some of the ceramic materials used to form molten metal filtration bodies and the processes for forming them.
Filters made from zirconia-alumina compositions have been found to have particular utility in filtering molten steel. This is because the filter bodies exhibit excellent physical properties such as high strength, high thermal shock resistance, and high resistance to corrosion at elevated temperatures. One limitation on the use of zirconia-alumina compositions however has been the problem of how to form large filter parts. This problem stems from the fact that during the cool-down portion of a firing cycle zirconia transforms from a tetragonal phase to a monoclinic phase. During this transformation, there is an associated volume expansion on the order of 3-5%. This volume change creates small cracks (microcracks) around the transformed particles, and large cracks (macrocracks) when the zirconia is not transformed uniformly throughout the filter body. While a certain amount of microcracking is beneficial in creating resistance to thermal shock, macrocracking causes failure. Large filters, those greater than eight inches, are particularly prone to macrocracking because of the large temperature gradients induced in them during the firing cycle.
To prevent macrocracking, zirconia has been stabilized with calcia or magnesia. These compounds go into solid solution in the zirconia grains and prevent or reduce the transformation from tetragonal to monoclinic by forming a stable cubic zirconia phase. Unfortunately, these stabilizers do not work well in the presence of alumina. This is because both calcia and magnesia come out of solid solution and react with the alumina rather than the zirconia.
Accordingly, it is an object of the present invention to provide a zirconia-alumina composition which is less prone to the formation of macrocracks.
It is a further object of the present invention to provide a zirconia-alumina composition which may be used to form relatively large parts.
It is yet a further object of the present invention to provide a ceramic body suitable for use as a molten metal filter which is formed from the above zirconia-alumina composition.
Further objects and advantages of the present invention will become more apparent from the following description.